1. Field of the Invention
The present invention relates to a joint structure and an insulating spacer applied for connection of a superconducting cable. More particularly, the present invention relates to a structure connecting a superconducting cable terminal to a central conductor of an insulating spacer for use in insulating the superconducting cable terminal from a grounded portion, at a terminating portion of the superconducting cable or at a connecting portion of two superconducting cables, and the insulating spacer for use in such connection.
2. Description of the Background Art
FIG. 6 is a perspective view of a representative example of a superconducting cable. The superconducting cable has a former 40 made of a metal wire at the core, and a superconducting layer (a superconducting wire) 41 covering former 40. Superconducting layer 41 generally consists of a tape-shaped superconducting wire 42 having superconducting filaments provided therein, which is wound around the former in a spiral fashion in a plurality of (in this example, four) layers, with the spiral pitches slightly changed from each other. Superconducting layer 41 is covered with a shield layer 44 via an insulating layer 43.
Shield layer 44 is for shielding the electric field generated from superconducting layer 41. Shield layer 44 generally consists of a tape-shaped superconducting wire having superconducting filaments provided therein, as in the case of superconducting layer 41. The tape-shaped wire is wound spirally in a plurality of (in this example, two) layers, with the spiral pitches slightly changed from each other. A protecting layer 45 further covers shield layer 44.
FIG. 7 shows a representative example of a terminating portion for connecting a superconducting cable for electric power transmission to a current lead for drawing out a current to an external current system. At a terminal portion 53 of the superconducting cable, the protective layer, the insulating layer and others of superconducting cable 51 are removed and replaced with a supplementary insulating paper 52. Provided further outside is a coolant 58 such as liquid nitrogen for cooling superconducting cable terminal portion 53. Superconducting cable 51 is connected to current lead 55 through a conductor that is connected to its terminal portion 53. This connection is secured inside a lower electric shield 54 to prevent leakage of the electric field to the outside.
Current lead 55 is covered with a bushing 56 made of FRP (fiber reinforced plastic) or the like to control leakage of the electric field. At an upper portion of the terminating portion, current lead 55 is further covered with porcelain insulator 57. The lower portion of current lead 55 and lower electric shield 54 are cooled with coolant 58 such as liquid nitrogen. A coolant tank 59 is provided within a vacuum chamber 60 for thermal insulation.
In such a terminating portion, coolant tank 59 and vacuum chamber 60 are grounded, which need to be insulated from superconducting cable terminal portion 53. An insulating spacer 61 is used for such insulation. Generally, insulating spacer 61 is made of epoxy resin (and thus called an “epoxy unit”), and is shaped thicker in the middle and tapered like cones at respective ends. A central conductor is provided at the core of insulating spacer 61 to let electricity flow therethrough.
The end of superconducting cable terminal portion 53 is connected to an end of this central conductor. The other end of the central conductor is guided into lower electric shield 54 and connected directly or indirectly to current lead 55.
Although there has not yet been presented any joint structure of a central conductor of an insulating spacer and an end portion of a superconducting cable, in an OF cable, CV cable or the like of normal conducting type, the central conductor and the cable end are normally connected by compression via a compression sleeve.
The above-described technique is disclosed in the following document:
Masuda et al., “Verification Tests of 3-core 66 kV HTS Cable System for Practical Use”, The Papers of Joint Technical Meeting on Application of Superconductivity and Linear Drives, IEE Japan, The Institute of Electrical Engineers of Japan, Jan. 24, 2002, Document No. ASC—O2-4, LD-O2-4, page 20.